1. Field of the Invention
The present invention relates to a rotary detector such as a rotary encoder. Particularly, the invention is desirably applicable to a rotary encoder for measuring the movement information of a diffraction grating, that is, the movement information of a moving object (scale), in such a manner that a coherent luminous flux such as laser light is incident on the diffraction grating or other minute grating array mounted on the scale to allow diffracted light of given orders from the diffraction grating to interfere with each other for the formation of interference fringes and measure the bright-dark fringes of the interference fringes.
2. Related Background Art
As a measuring instrument, there has hitherto been a rotary encoder widely used in many fields, which is capable of measuring the rotational information such as rotational amounts and rotational directions of a rotating object in an NC machine with high precision, for example, in the unit of submicron orders.
Particularly, as a highly precise, high resolution rotary encoder, there is well known a rotary encoder of a diffraction light interference type wherein a coherent luminous flux such as laser light is allowed to enter the diffraction grating provided at a moving object to enable the diffracted light of given orders emitted from the diffraction grating to interfere with each other and obtain the movement amounts, movement directions, and other data of the moving object by measuring the brightness-darkness of the interference fringes.
FIG. 1A is a view schematically showing the principal part of a portion of a conventional rotary encoder of a diffracted light interference type.
In FIG. 1A, the monochromatic luminous flux which is emitted from a light source 101 enters a minute grating array 105 having a grating pitch P (the number per round of the diffraction grating array being N) composed of the diffraction grating and others on a scale (disc) 105a to emit a plurality of diffracted lights. In this case, the order of luminous flux advancing linearly is defined as zero. On both sides thereof, diffracted lights having orders such as .+-.1, .+-.2, .+-.3, . . . are defined. Further, the rotational direction of the scale 105a is distinguished by providing it with a mark "+" from its reverse direction for which a mark "-" is provided. Then, the rotational angle of the scale 105a is given as .theta. (deg.) with respect to the wave surface of zeroth light, so the phase of the wave surface of n-th diffracted light is shifted by: EQU 2.pi..multidot.n.multidot.N.multidot..theta./360.
Now, since the wave surface phases of the diffracted lights having different orders are shifted from each other, it is possible to obtain brightness-darkness signals by superposing the optical paths of two diffracted lights with an appropriate optical system to allow them to interfere with each other.
If, for example, using mirrors 109a and 109b, and a beam splitter 103, a + primary diffraction light and a - primary diffraction light are superposed to interfere with each other, their phases are displaced 4.pi. while the scale 105a is rotated by one pitch portion (360/N degrees) of the minute grating. Hence, there occurs the change in the light amount of the brightness-darkness for two cycles. Consequently, if the change in the light amount of the brightness-darkness is detected at this juncture, it is possible to obtain the rotational amount of the scale 105a.
FIG. 1B is a view schematically showing the principal part of a portion of a conventional rotary encoder of the diffracted light interference type capable of detecting not only the rotational amount of the scale 105a but also the rotating directions thereof.
In FIG. 1B, as compared with the rotary encoder shown in FIG. 1A, there are prepared at least two kinds of brightness-darkness signals obtainable from the two diffracted lights accompanied with the rotation of the scale 105a, and the rotating direction of the scale 105a is detected by shifting the timing of brightness-darkness thereof from each other.
In other words, according to FIG. 1B, before the n-th diffracted light and the m-th diffracted light emitted from the minute grating array 105 are superposed, both of them are made into luminous fluxes linearly polarized, whose polarized wave surfaces are orthogonal to each other, by utilizing polarization plates 108a and 108b. Then, after the optical paths are superposed through mirrors 109a, 109b and a beam splitter 103a, the luminous fluxes pass through a 1/4 wavelength plate 107a, and thus are transformed into the linearly polarized waves in which the orientation of the polarized wave surface is determined up to the phase difference between the two luminous fluxes.
Further, these waves are divided into two luminous fluxes by a non-polarized beam splitter 103b. Each of the luminous fluxes is transmitted through each of the polarization plates (analyzers) 108c and 108d which are arranged so as to shift the detection orientations (the orientations of the transmittable linearly polarized light) of the luminous fluxes from each other. Hence, the two kinds of brightness-darkness signals whose brightness-darkness timing is shifted due to the interference of the two luminous fluxes are detected by the detectors 110a and 110b.
If, for example, the detection orientations of these two polarization plates are deviated 45.degree. from each other, the brightness-darkness timing is shifted 90.degree. (.pi./2) in terms of phase. At this juncture, the rotary encoder shown in FIG. 1B detects the rotational information including the rotating direction of the scale 105a using the signals from the two detectors 110a and 110b.
Now, such a rotary encoder obtains interfering signals by interfering the diffracted light once diffracted by the diffraction grating, and then obtains rotational information by detecting such signals. In order to enhance the detection resolution, however, it is desirable to allow diffracted light to be diffracted twice to interfere with each other. Also, in order to avoid any influence or the like from the eccentricity of a scale, it is desirable to perform this two-time diffraction at points as far apart as possible (optimally, at two points substantially point symmetrical with respect to the rotational center).